Caledonian-Indosinian Mineralization In The North Wuyi Area

The North Wuyi area of Jiangxi Province,China,is an important part of the Wuyishan metallogenic belt and is endowed with significant amounts of copper-lead-zinc polymetallic deposits.A lot of previous studies have been carried out on the ore genesis of these deposits in the Wuyishan metallogenic belt,showing a typical characteristic of large-scale mineralization during Yanshanian.Therefore,most of the previous research has been focused on the Yanshanian magmatism and mineralization,with little attention have been paid to the relationship between Caledonian and Indosinian magmatism and mineralization.It has been demonstrated that intensive magmatic activities are always accompanied by the mineralization.In the North Wuyi area,it is still unclear whether the mineralization associated with Caledonian and Indosinian magmatism occurred or not.In this study,we select the Changshouyuan Pb-Zn-Ag deposit with an intimate spatial relationship to the Caledonian magmatism and the Chenfang Cu-polymetallic deposit with a closed spatial relationship to the Indosinian magmatism,respectively,trying to unravel this issue.Based on the detailed field investigation and indoor petrographic observation,geochronologic studies are conducted to precisely constrain the age of the deposits to verify the mineralization fertility of the Caledonian and Indosinian magmatism in this area.In addition,systematic studies on the source of the metals and ore-forming fluid are also conducted to reveal the ore-forming process and ore genesis.Integrated previous studies and our studies would improve the understanding of the age of mineralization and metallogeny of the copper-lead-zinc polymetallic deposits in North Wuyi area,and provide scientific theoretical guidance for the mineral prospecting and exploration.The Changshouyuan Pb-Zn-Ag deposit is hosted in the interbedded siliceous fracture zone of the upper unit of Sinian Hongshan formation.The mineralization has a closed relationship with the Medium-to fine-grained dilotic monzonitic granite.The hydrothermal alteration mainly consists of silicification and sericitization.The paragenesis can be divided into three stages,including:quartz-pyrite-sphalerite stage,quartz-pyrrhotite-sphalerite-pyrite stage,and quartz-pyrite-sphalerite-galena stage.The Chenfang copper polymetallic deposit is hosted in the outer contact zone between Carboniferous Outangdi formation and biotite granitoid of Liangjia unit and is structurally controlled by the faults.The mineralization can be divided into skarn stage and quartz-sulfide stage.The skarn stage can be subdivided into prograde alteration skarn stage and retrograde alteration skarn stage.The quartz-sulfide stage can be subdivided into quartz-fluorite-sulfide stage and fluorite-calcite-sulfide stage.The granite in the Changshouyuan Pb-Zn-Ag deposit is high-K and calc-alkaline with strong peraluminous properties.Trace element composition shows a depletion in Ba,Nb,Sr,and Ti,and an enrichment in Rb,Th,and U.The chondrite-normalized REE patterns is characterized by evident negative Eu anomaly and LREE-enriched,with the obvious fractionation between LREE and HREE.The granite yields a zircon age of 431±19Ma.The hydrothermal apatite intergrown with sphalerite and pyrite in the main ore stage has a U-Pb age of 438.4±4.9Ma,which is consistent with the age of the granite within the error range.The results show that the Changshouyuan Pb-Zn-Ag mineralization and granite magmatism are coeval at Caledonian,which is the first time reported in the North Wuyi area and demonstrates the great potential of Caledonian mineralization associated with the Caledonian magmatism in this area which should be pay more attention in the future study and exploration.The microthermometry of the fluid inclusions in the quartz shows that stage I has the homogeneous temperature varying from 279～406°C（mean 337.7°C）,with the salinity ranging from2.74～9.47wt.%Na Cleq.（mean 6.01wt.%Na Cleq.）.At 330 to 337°C,the boiling of the fluid occurred.The stage II has the homogeneous temperature varying from168～357°C（mean 277.8°C）,with the salinity ranging from 2.57～9.98wt.%Na Cleq.（mean 5.67wt.%Na Cleq.）.The stage III has the homogeneous temperature varying from 166～328°C（mean 239.9°C）,with the salinity ranging from2.57～8.14wt.%Na Cleq.（mean 4.99wt.%Na Cleq.）.The Laser Raman spectrometry analysis shows that the vapor is composed of CH₄,H₂S,CO₂,and minor amount of N₂.Hydrogen and oxygen isotope analysis of quartz shows that theδ¹⁸O value ranges from5.76 to 9.32‰and theδD_V-SMOWvalue ranges from-69 to-57‰,which is characteristic of magmatic-hydrothermal fluid,indicating the ore-forming fluid in Changshouyuan Pb-Zn-Ag deposit is of magmatic-hydrothermal origin,which is medium-high temperature,low salinity and density,reduced,and composed of Na Cl-H₂O±CO₂±CH₄±H₂S.The carbon isotope compositions of the fluid inclusion vary from-2.0 to 0.3‰,indicating a marine carbonate origin,and can be interpreted to be resulted from the extraction of the marine carbonate by the magmatic-hydrothermal fluid.The sulfur isotope composition of the sulfide in stage I（average 6.4‰）is consistent with sedimentary pyrite（average 7.3‰）in the host rock,and is much higher than the sulfide of stage II（average 5.0‰）and stage III（average 2.3‰）.The decreased sulfur isotope compositions imply that metal in the stage I is derived from the sedimentary rocks,with gradual input of magmatic-hydrothermal fluid along with the mineralization,as indicated by the sulfur isotope compositions of sulfide from stage I to stage III.Trace element composition of the sphalerite is indicative of a magmatic-hydrothermal origin.In the binary diagram of Ln（Ln）and Ln（Ga）,all the plots are bracketed in the magmatic-hydrothermal area.Additionally,Cd concentration（7183～9681ppm）of the sphalerite in all stages is above the industrial grade,which is obviously higher than that in general magmatic-hydrothermal deposits.These geochemical signatures indicate that Cd was extracted by the ore-forming fluid from the strata during the mineralization.Silver content（387～2364ppm）in the chalcopyrite is obviously higher than the content in pyrite and sphalerite,which is characterized by a trend of progressive increase from early to late stage,indicating Ag is mainly hosted in the chalcopyrite and predominantly precipitated in the late stage.Collectively,the formation of the Changshouyuan Pb-Zn-Ag deposit is via the following process:Pb,Zn,Ag and Cd in the Sinian Hongshan Formation was reactivated and extracted by the magmatic-hydrothermal fluid in Caledonian,along with the significant input of magmatic-hydrothermal origin.The abrupt decompression and boiling and resultant decrease of the p H and temperature of the ore-forming fluid is the main mechanism responsible for the precipitation of the metals.Therefore,Changshouyuan belongs to a magmatic-hydrothermal Pb-Zn-Ag deposit which is controlled by the interbedded fracture zones.The biotite granite in the Chenfang copper polymetallic deposit is high-K and calc-alkaline with strong peraluminous properties.Trace element composition is depleted in Ba,Nb,Sr,P,and Ti,but enriched in Rb,Th,U,and Pb.The chondrite-normalized REE pattern shows a negative Eu anomaly,LREE-enriched,and obvious fractionation between LREE and HREE.The biotite granite and K-feldspar granite have zircon U-Pb age of 226.4±1.5Ma and 227～224Ma,respectively,consistent with the U-Pb age of cassiterite of 223±2.5Ma in the stratiform magnetite and U-Pb age of cassiterite of 231±3.5Ma in the massive magnetite within the error range,indicating that the Chenfang copper polymetallic mineralization and granite magmatism are coeval,demonstrating the presence of Indosinian mineralization in the North Wuyi area.TheεHf_（t）value of zircon ranges from-12.0 to-5.6,with the model age of Hf ranging from 1998 to 1591 Ma,indicating the biotite granite is a product of partial melting of Proterozoic crust.The oxygen isotope composition of the magnetite in the late ore-stage quartz-calcite-fluorite veins varies from-3.8‰to-2.3‰,which is much lower than the values of the magnetite（2.1‰to 3.4‰）in the early skarn stage,indicating that magmatic-hydrothermal fluid was mixed by a certain proportion of meteoric water characterized by the low oxygen isotope composition.In situ sulfur isotope analysis shows that the sulfur isotope composition of the sulfide in stage I（mean=-1.3‰）is much higher than stage II（mean=-9.1%）,implying a muti source of the metals.The metals are mainly derived from the biotite granite,with possible input of meteoric water in the late ore stage,as well as other sulfur into the hydrothermal system.Trace element composition of pyrite in the stage I is enriched in Cu,Zn,Bi,and As,with the stage II enriched in Au,Pb,Mn,and Sn in which Au occurs mainly as native gold particle,indicating the metal source of stage I is different from stage II.Results presented here suggest that magmatic-hydrothermal fluid is the main ore-forming fluid in the Chenfang opper polymetallic deposit,and mixed with a certain proportion of Au,Pb,Mn,and Sn and meteoric water in the late ore stage.Collectively,Chenfang is a typical skarn copper polymetallic deposit.In summary,the North Wuyi area involved Caledonian and Indosnian metallogenies.The integrated geological and metallogenic characteristics,temporal and spatial evolution of mineralization,ore-controlled factors,and the mechanism responsible for the lithogeny and mineralization in the North Wuyi area show that the copper-lead-zinc polymetallic mineralization can be divided into five periods:（1）Mesoproterozoic to Neoproterozoic submarine volcanic activity and sedimentary exhalative activity formed the submarine hydrothermal exhalative sediments associated with spilite and baschtauite as well as later hydrothermal-magmatic overprinted and modified copper polymetallic deposits,which is represented by Tieshajie deposit.（2）In Silurian（438Ma）,intensive tectonic and magmatic events occurred in the North Wuyi area and were accompanied by the strong regional metamorphism,with the resultant formation of magmatic-hydrothermal Pb-Zn-Ag deposits associated with strong peraluminous granite,such as Changshouyuan Pb-Zn-Ag deposit.In this stage,graphitization and Cd mineralization are also observed.Mineralization in this stage was likely to be destroyed and modified by the strong Caledonian（430-490Ma）tectonic activities.（3）Middle to late Triassic（235-226Ma）,the skarn and hydrothermal vein-type copper polymetallic deposits associated with post-collision eraluminous granite formed.These deposits are enriched in Fe,Cu,Pb,Zn,and minor Sn,such as Chenfang（226Ma）,Gaojiashan（232Ma）,and Xiahu deposits（235Ma）;（4）In late Jurassic（160-154Ma）,the subduction of Paleo Pacific block beneath the Eurasian resulted in the formation of series of NE-striking lithospheric stretching belts and deep faults,and formed the skarn and porphyry copper polymetallic deposits associated with porphyritic granite which are enriched in Cu,Mo,W,Pb,and Zn,such as Lengshuikeng deposit（160-157Ma）,Yongping deposit（160Ma）,and Longtougang deposit（154Ma）;（5）In early Cretaceous（139-136Ma）,the large-scale northward strike slip of the Pacific block along the NEE-trending strike-slip fault resulted in the formation of continental extension structure regime.This extensive extension promoted the development of NE-striking rifted basin and resultant formation of volcanic-subvolcanic hydrothermal Pb-Zn-Ag deposits,such as Jinzhuping deposit（136Ma）,Chengshuping deposit（139Ma）,Caijiaping deposit（137Ma）,and Shengmikeng deposit（137Ma）.